1. Field Of The Invention
The present invention relates to integrated circuit technology and integrated circuit fabrication techniques. More particularly, the present invention relates to incorporation of a low voltage MOS device in a high voltage substrate or well.
2. The Prior Art
The design of MOSFET and MESFET circuits sometimes requires the inclusion of both low voltage and high voltage circuits on the same chip. Inclusion of both types of circuits is sometimes needed for aspects of the operation of a single chip, such as low voltage circuits for reading a PROM and high voltage circuits for programming the PROM. The design of MOSFET or MESFET circuits which include an interface between low voltage circuits and high voltage circuits traditionally incorporates two separate wells, one well for low voltage devices and a second well for high voltage devices. These separate wells are usually surrounded by double guard rings to suppress destructive latch up. The double wells and their attendant guard rings constitute a considerable layout area penalty, especially for repetitive circuits such as address or decoder level shifters.
As integrated circuit technology matures, and as circuit and layout designers strive to increase the density of integrated circuits, the provision of separate wells and their attendant guard rings for high and low voltage devices creates a limitation on achievable circuit density on an integrated circuit chip.